1. Field of the Invention
The present invention relates to an area machining method, and more specifically, to an area machining method for hollowing out an area whose boundary is partially open.
2. Description of the Related Art
Area machining is a type of machining that hollows out an area arbitrarily defined on a surface of a workpiece, thereby forming a pocket. Part programs for the area machining are obtained by using an automatic programming system or a CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) system. For example, when using an automatic programming system to create a tool path to hollow out a specific area, it is necessary to specify the final shape of the area to be cut and finished as well as the cutting conditions. The cutting conditions typically include such data as tool radius, cutting feedrate, depth of cut, and finishing allowance. The depth of cut and the finishing allowance should be defined in both radial and axial (i.e., Z-axis) directions.
Conventional area machining methods assume that they will finally produce a continuous wall along the boundary of the area, and the machined area (or pocket) will be "closed" by the wall.
Actually, however, it is often required to machine an area that is not closed by a wall unlike the one described above. In other words, a part of the pocket produced by the machining is open on a side of the workpiece. In this sense, this kind of area is to be called a "partially open area" in this description. The conventional area machining methods have been applied also to this partially open area, although they were originally developed for the aforementioned closed area.
FIG. 8 is a diagram showing a conventional area machining method for hollowing out a partially open area. The reference numeral 1 indicates a workpiece, and 2-1, 2-2 and 2-3 represent the boundary that specifies the area. The machine tool will hollow out the inside of the area bounded by those three line segments and a line segment 2-4 which is a part of the workpiece outline.
In reality, however, the conventional methods request to define the area as indicated by a broken line 3 instead of the line segment 2-4. That is, the area should be defined as if it hanged out of the workpiece 1. This is for the reason that, in every cutting cycle of the area machining operation, the tool should be once moved out of the workpiece not to leave any part unmachined.
Based on the contour of the area defined by the lines 2-1, 2-2, 2-3 and 3, the automatic programming system generates a cyclic tool path to enlarge the open pocket by repetitively feeding the tool by a constant depth of cut from inside to outside. In FIG. 8, narrow lines 4-1, 4-2 and 4-3 indicate the offset contours, which are generated from the contour of the area, each offset contour having a different offset distance therefrom. The complete tool path will be created by connecting those offset contours in the order of 4-3, 4-2 and then 4-1 (i.e., from inside to outside).
FIG. 9 shows another example of the conventional machining method for hollowing out a partially open area. The figure illustrates a rectangular solid workpiece 1, one of whose corners will be machined as indicated by lines 2-1 and 2-2.
In this case, a contour machining method is used for generating a tool path by the following simple steps. First, offset contours 4-1, 4-2 and 4-3 are created by repeating an outward displacement of the target contour defined by the lines 2-1 and 2-2. Next, the tool path is created by connecting those offset contours so that the tool will proceed in zigzag along the offset contours toward the target contour (i.e., from 4-3 to 4-2 and then 4-1).
However, according to those conventional methods of area machining, a substantial portion of the generated tool path will run in the space where no workpiece exists. As a result, the tool will continue cutting "air" at a constant cutting feedrate even though it has already left the workpiece.
Further, the depth setting motion in each cycle is done also at this low feedrate. Although the tool could traverse to the next starting position at a higher speed, it must move at a low cutting feedrate in the conventional area machining. This so-called "air cut" operation makes the total machining time longer, thus degrading the productivity of area machining.